Various methods for generating bubbles have already been proposed. Examples in this regard are a) gas transport methods in which a gas is passed through the micropores of a gas dispersing tube into a liquid; b) methods in which a vibration with a frequency no greater than 1 kHz is applied to a porous body while a gas is being fed into a liquid through the porous body; c) bubble generation methods that utilize ultrasound; d) shaking•stirring methods in which bubbles are generated by stirring a liquid and shearing a gas; e) methods in which a gas is dissolved under pressure in a liquid followed by pressure reduction in order to generate bubbles from the supersaturated dissolved gas; and f) chemical foaming methods in which bubbles are created by generating a gas in a liquid by a chemical reaction (refer, for example, to Clift, R. et al., “Bubbles, Drops, and Particles”, Academic Press (1978), and Hideki TAKUSHOKU, “Progress in Chemical Engineering. 16. Bubble, Drop, and Dispersion Engineering”, Maki Shoten, 1 (1982)).
However, these methods, excluding methods that generate microfine bubbles utilizing microwaves, not only have difficulty producing very fine bubbles with bubble diameters on the order of nanometers, but also suffer from the problem of an impaired stability due to a nonuniform bubble diameter. In addition, it is also extremely difficult in the aforementioned methods to freely adjust the bubble diameter.